70 Fundamentals of Auxin Action 



material by Kogl and Kostermans (1934) who identified it in yeast. 

 It has also been positively identified in corn seedlings (Haagen-Smit 

 et al, 1946), and indirect evidence has indicated that it is the major 

 growth hormone in the organs of seven other growing plants: bean 

 stems (Verkaaik, 1942), sugar cane stems (van Overbeek et al, 1945), 

 potato tubers (Hemberg, 1947), Avena coleoptiles (Wildman and 

 Bonner, 1948), pineapple leaves (Gordon and Nieva, 1949), and 

 tomato stems (Kramer and Went, 1949). In each of these plant mate- 

 rials several different criteria have been used to establish that 

 indoleacetic acid is the principal growth hormone. By the use of 

 chromatography, auxins have been separated from plants and shown 

 to have Rf values identical with indoleacetic acid in many additional 

 species of plants, including cabbage (Holley et al, 1951), Brussels sprouts 

 (Linser, 1951), and cauliflower (von Denffer et al, 1952). 



There is evidence for the existence of the ethyl ester of indole- 

 acetic acid in corn kernels (Redemann et al, 1951) and in apple endo- 

 sperm (Teubner, 1953). The ester form is less effective in promoting 

 growth than the free acid, but more effective than the acid in causing 

 fruit set. This reported difference may be attributed to greater pene- 

 tration or persistence of the ester rather than to better auxin proper- 

 ties per se. 



The discovery of auxins a and b, originally isolated from human 

 urine, has been described in an earlier section. There is some indirect 

 evidence, principally in terms of molecular weight determinations, 

 enzymatic inactivation and stability to acid and to alkali, for the 

 presence of these compounds in plants. However, repeated attempts 

 to find them in growing plants have failed. 



Indolepyruvic acid has been identified in plants by chromato- 

 graphic techniques (Stowe and Thimann, 1953). This compound is 

 known to have auxin activity. It is also readily broken down into 

 indoleacetic acid, so the extent of its role as a growth hormone is 

 difficult to determine. 



Investigations of callus tissues resulting from the wounding of 

 various plants have led to the identification of traumatic acid as a 

 growth promoting substance associated with tissue proliferation 

 around wounds. Traumatic acid is a straight chain dicarboxylic acid: 

 HOOCCH = CH (CH2)8COOH. It lacks the unsaturated ring charac- 

 teristic of all known auxins. As far as is known, traumatic acid is not 

 active as an auxin, but seems to act in conjunction with auxins to 

 produce the wound growth response (see review by Thimann, 1948, 

 p. 84). 



Three other compounds which show auxin activity have been 



